Effect of Pacing Site on the Atrial Electrogram at Target Sites for Slow Pathway Ablation in Patients with Atrioventricular Nodal Reentrant Tachycardia JOHN HUMMEL. S. ADAM STRICKBERGER, STEVEN KALBELEISCH, BRIAN WILLIAMSON, K. CHING MAN, VICKEN VORPERIAN, ERED MORADY, and JONATHAN LANGBERG From the Division of Gardiology, University of Michigan Medical Center, Ann Arbor, Michigan HUMMEL, J., ETAL.: Effect of Pacing Site on the Atrial Electrogram at Target Sites for Slow Pathway Ahlation in Patients with Atrioventricular Nodal Reentrant Tachycardia. Atrial eiectrograms recorded from target sites during radiofrequency catheter ablation of the slow atrioventricular (AV) nodal pathway are often fractionated and may be associated with a late, high frequency component (the slow pathway potential). The purpose of the current study was to assess the effects of slow pathway ablation on the morphology of the atrial electrogram and to determine whether target site eiectrograms display direction dependent changes in morphology during atrial pacing maneuvers. Twenty-six patients with typical AV nodal reentry had eiectrograms recorded from target sites before and after successful ablation of the slow A V nodal path way and during pacing from the high right atrium and distal coronary sinus at cycle lengths of 500 and 300 msec. There was no significant change in the duration or degree of fractionation of the atrial electrogram as the result of slow pathway ablation. In contrast, the duration and degree of fractiona- tion were less when pacing from the coronary sinus compared with sinus rhythms or right atrial pacing. Pacing rate did not affect electrogram morphology. These data suggest that the morphology of the slow pathway target site electrogram is dependent on the direction of atrial activation and that the "slow pathway potential" does not represent activation of an anatomically discrete pathway. (PACE, Vol. 17, April 1994, Part 1) radiofrequency catheter ablation, supraventricular tachycardia, anisotropic conduction Introduction Multicomponent atrial eiectrograms are use- ful as a guide for radiofrequency ablation of the slow pathway in patients with typical atrioventric- ular nodal reentrant tachycardia (AVNRT). These target site eiectrograms have been analyzed in an attempt to define the functional and anatomical Address for reprints: John Hummel, M.D.. tJniversity of Michi- gan Medical Center, Division of Cardiology, 1500 East Medicial Center Drive, Ann Arbor, MI 48109-0022. Fax: (313) 763-7390. ReceivRd June 18, 1993; reviyion October 27, 1993; accepted December 6, 1993. properties of the slow pathway.^ ' A prior study examined the effect of rate, but none have exam- ined the effect of pacing site on electrogram mor- phology. Therefore, the purpose of this study was to determine whether target site eiectrograms dis- play direction dependent changes in morphology during atrial pacing maneuvers. Methods Characteristics of Suhjects The subjects of this study were 26 patients with typical AVNRT who underwent successful radiofrequency ahlation of the slow pathway. Pa- PAGE, Vol. 17 April 1994, Part I 585